The use of solar radiation for heating fluids, such as water, has been known for many years. Modern solar heating systems incorporated into buildings typically include one or more solar energy collector panels suitable for absorbing energy from the sun wherein the solar energy is converted into thermal energy transferred to fluids circulating therethrough and which subsequently circulate throughout the buildings for heating purposes or for storage.
Solar energy collector panels are generally installed on the roofs of buildings. A small residential home will typically require a solar system having at least about three or four square meters of solar energy collection surface area to supplement domestic water heating. A home that also uses solar energy for space heating or other heating applications will typically require ten to twenty square meters of collection surface area whereas an industrial or commercial facility may need thirty square meters or more depending on the solar heating application.
Solar collector panels are usually mounted flat on roofs to be the least obtrusive to the overall appearance of the building. Such configurations do not maximize the amounts of solar energy that may be captured by the collectors. In more northern regions, the solar energy collectors should be tilted up towards the southern horizon, and in more southern regions, they should be tilted up towards the northern horizon. Tilting solar collector panels increases the capture of solar energy in winter months, but this can cause wind load problems, especially with flat-plate collectors. Evacuated-tube collectors are not as prone to wind loading due to spacing provided between the tubes. Tilting of the solar energy collector panels, however, results in prominent views of the solar energy collectors which may be undesirable. One solution has been to mount the solar energy collectors on a south-facing wall (in northern hemispheres). Such installations are possible during new building construction, but may be difficult or impossible for retrofitting existing buildings. Other solutions for mitigating the visual prominence of solar collector panels are achieved by architecturally integrating the collector area into the building structure. Examples of architecturally integrated solar collectors include window shutters, balcony railings, awnings, facia, fences and privacy screens.
Solar water heaters known in the art generally comprise one of two systems. “Combined systems” have one or more solar energy collector panels mounted directly onto a water storage tank. Combined systems are generally used in warm climates because the water tanks are exposed to the ambient environment and consequently, fluids stored therein are rapidly cooled as ambient temperatures drop. “Distributed systems” have solar energy collector panels mounted on surfaces receiving solar radiation and are connected by piping infrastructures to water storage tanks located in sheltered spaces to minimize heat losses due to cooler ambient temperatures.
Additionally, solar water heaters are classified as either “direct systems” or “indirect systems”. Direct systems can comprise combined or distributed systems, and are configured to circulate domestic water through the solar energy collector panels. One problem with direct systems is that circulation of domestic water often causes the formation of mineral scales along the interior surfaces of piping comprising the solar energy collector panels. Another problem with direct systems is that solar energy collector panels must be drained when ambient temperatures fall below freezing to prevent damage to the piping. Indirect systems are more sophisticated distributed systems that circulate a heat transfer fluid, also commonly referred to as a working fluid, between the solar energy collector panels through a piping infrastructure to a heat exchange unit communicating with the working fluid on one side, and a flow of potable water on the other side. The heat exchange unit transfers thermal heat from the working fluid to the potable water. The heat exchange unit may be placed inside a water tank wherein potable water is stored. Alternatively, the heat exchange unit may be located on an external surface of the water storage tank and directly connected thereto, or further alternatively, the heat exchange unit may be located in a separate location and connected to the water tank by a piping infrastructure. Working fluids used in the indirect systems typically are provided with antifreeze agents.
The above classification of solar water heaters also applies to solar heaters used in other heating applications such as pool heating, space heating, process heating or any other application where heating a fluid is desired. In all cases a fluid heated by solar energy collector panels is used directly or indirectly to provide heat energy.
Conventional solar heating systems are usually configured to enable stoppage of the circulation of the potable water or working fluid through the piping infrastructure connected to the solar energy collector panels, the water storage tanks and the heat exchange units during periods of low or no solar irradiation in order to avoid the loss of thermal energy that has been previously captured by the solar collector panels and reduce unnecessary pump operation costs. Some systems use a “drain-back” approach to drain the working fluid into a holding tank during shutdown periods. Non-“drain-back” systems require the addition of sufficient anti-freeze agent to ensure working fluids do not freeze and cause physical damage to the piping and/or the solar energy collector panels. Regardless of the shutdown strategy used, the working fluids cool significantly during idle periods and have to be re-heated once the system starts operating. This results in inefficient operation during the first few minutes or hours of operation of the solar thermal energy capture systems depending on how low the ambient temperatures become and the quantity of working fluid in the solar heating system. It is a particular problem in cold climates where overnight temperature drop significantly below freezing.
Solar thermal energy capture systems must also be configured to prevent damage that may be caused by excessive heat build-up during periods of high solar energy. Control methods are necessary to mitigate damage from high-temperature fluid flowing through either or both of the working fluid side and the load side of the heat exchange units and piping infrastructure. Additionally, as a fail-safe precaution, allowances must be made for relief of pressure that may accumulate through excessive heat on the working fluid side caused by extended intense solar radiation. Strategies used for controlling overheating in conventional solar thermal energy capture systems include among others: (i) “heat dumps” which are a remote heat application where unwanted heat is dumped such as under an outdoor patio; (ii) draining back and shutting down the system and permitting the solar collectors to stagnate; (iii) manually setting the system controller to a “vacation” setting when the primary application is not required, as is the case when a homeowner with a solar water heater goes on vacation, which activates a control sequence to avoid over heating such as by circulating the working fluid through the collectors over night to increase heat losses; or (iv) deliberately under-sizing the system such that the system's peak output provides 90% of the minimum anticipated heat load, which means that other energy providers exemplified by gas utilities and electric utilities, must be relied upon as supplemental heating sources.
Consequently, conventional systems configurations for solar thermal energy capture have become fairly complex. Some systems are designed to have multiple fluid circuits wherein one circuit is dedicated for dissipation of excess heat energy. Other strategies employ complex piping infrastructures with separate heat dissipater circuits for cooling overheated working fluid exiting the solar energy collector panels. With such systems, all of the working fluid is heated by the solar panels and then cooled after egress resulting in system inefficiencies.
Most conventional solar thermal energy capture systems incorporate electronic control systems for activating or de-activating the fluid circulation pumps and valves to enable control over and manipulation of fluid flow throughout the system, filling or imposing drain-backs, directing working fluid through solar collector panels and heat exchange units, or for activating over-heating mitigation mechanisms. These control and operating systems require electrical energy usually supplied by an electric utility company. Electrical power interruptions will result in loss of control over the solar thermal energy capture system that could result in physical damage to the system, and in collateral damage to the building in the event that the piping infrastructure fails. One solution to ensure electrical power supply has been to use battery backups. In some systems, solar photovoltaic (PV) systems are available to supply the necessary electrical energy either directly to the solar heating system and controller, or indirectly, such as battery packs and the like. However, battery systems have a relatively short life expectancy. Moreover, failure to test and replace the battery can lead to the same type of damage as seen with loss of electrical power. Furthermore, the electronic control methods are prone to component failures, particularly within the twenty- to thirty-year life expectancy of solar collector systems.